Pyotr Verzilov arriving on a special medical transport plane at Schönefeld airport in Berlin, on 15 September.
Photograph: Reuters

The Pussy Riot activist Pyotr Verzilov who fell ill in Moscow was almost certainly poisoned, German doctors treating him in Berlin said on Tuesday, adding that he may have been the victim of an unknown “anticholinergic agent”.

It was “highly probable” Verzilov’s dramatic symptoms – which include amnesia, loss of vision and being unable to walk or talk – were externally induced, the doctors said.

Verzilov was one of four members of Pussy Riot who invaded the pitch dressed in police uniforms during the football World Cup final in Moscow in July. They were protesting against excessive Russian police powers.

Verzilov felt abruptly unwell after a court hearing last Tuesday. He is the latest in a long line of Kremlin critics and opposition activists to have been apparently poisoned in murky circumstances, both at home and abroad.

He was initially treated in Moscow and then rushed to Berlin on Saturday night. Video posted by his ex-wife Nadia Tolokonnikova who travelled with him showed Verzilov arriving in a dazed and weak condition and carried from a private plane.

“It is highly probable that he was poisoned,” Dr Kai-Uwe Eckardt of Berlin’s Charitéhospital told a news conference. He said he came to this conclusion based on information from relatives and the Moscow hospital where he was treated last week, plus symptoms such as disorientation and widened pupils.

Verzilov appeared to be suffering from anticholinergic syndrome – the disruption of the nervous system that regulates the inner organs. The cause was a toxin “which we haven’t identified yet and may not be able to identify,” Eckardt’s colleague Karl Max Einhäupl said.

She said he was disorientated, dizzy, confused and “not fully with us as the Peter we know”.

“He remembers his friends and relatives, but he does not understand that he’s in Germany, that he’s in a hospital and there are doctors around him, not prison wardens,” she said.

Tolokonnikova said her ex-husband’s thoughts were “jumping from one subject to another quickly” and that he was obviously suffering from amnesia; “in the very beginning he could not recognize his mother.”

Despite his ordeal Verzilov retained his “unique sense of humour”. He had said to her and Nika Nikulshina – Verzilov’s girlfriend who flew with him to Berlin: “So good to see you without handcuffs.”

Members of Pussy Riot are convinced Verzilov was deliberately poisoned, as part of an attempt to intimidate or even murder him. They said the effects of the toxin suggested it was from a group of “40 or 50” anticholinergic agents. These were ideal for poisoning someone, since they quickly disappear from blood and urine, leaving the compound involved a mystery.

“It’s important to realise that Peter’s life was in danger … In large doses anticholinergic drugs can cause respiratory failure and death,” the collective said.

Moscow has a long tradition of poisoning state enemies. In 1921 Vladimir Lenin established a secret poisons factory, which operated throughout the Cold War. It is still in business, western governments believe, and is housed in an ordinary looking beige building outside Moscow called Scientific-Research Institute No. 2.

Assassins allegedly sent by the Kremlin’s spy agencies have tracked down and poisoned dissidents living in the UK. They include Georgi Markov – killed in 1978 with a ricin pellet fired from a modified umbrella – and Alexander Litvinenko – murdered in 2006 with a radioactive cup of green tea.

The former Russian spy Sergei Skripal and his daughter Yulia were targeted in the UK in March with the deadly nerve agent novichok. Theresa May has accused two GRU military intelligence officers, Alexander Petrov and Ruslan Boshirov, of carrying out the Skripal hit.

Two days after Verzilov collapsed in Moscow, Petrov and Boshirov appeared on the Kremlin-controlled RT channel. They admitted visiting Salisbury twice during the weekend of the poisoning but said they had gone to the town to see the cathedral. Downing Street dismissed their explanation.

Verzilov is a prominent member of the anti-Kremlin scene and has been involved in protest art in Russia for more than a decade. His flight to Berlin was organised by the Cinema for Peace Foundation, which has close connections with Pussy Riot and is based in the German capital.

Writing on Facebook at the weekend, Verzilov’s girlfriend and fellow Pussy Riot member Veronika Nikulshina expressed relief that he had been transferred out of Russia.

“Three times hurrah to everyone who wrote, phoned, visited, cried and sang. We’re in Berlin. All is well,” she said.

Tests were unable to determine what had poisoned Kara-Murza, though doctors concluded he had been the victim of two “binary” poisons, possibly introduced separately. Kara-Murza had campaigned in the US to impose sanctions on the Putin regime and was a colleague of the opposition leader Boris Nemtsov, shot dead in February 2015 outside the Kremlin.

It is unclear if and when Verzilov will return to Moscow. Tolokonnikova said on Tuesday that “his life is still in danger in Russia”.